The present invention relates to a circuit arrangement for digital indication of the received frequency in a radio receiver operating in the AM and FM ranges, wherein the oscillations of the AM and FM oscillators are amplified, converted to rectangular oscillations and counted by means of electronic counters.
It is known that in radio receivers the indication of frequency of the transmitting station is effected by a pointer which is driven by the tuning member, in most cases a variable capacitor. via a rope pulley or other similar mechanical devices. The accuracy of this indication, aside from that in commerical receivers, is unsatisfactory, however. With this type of system, the error amounts to a few kHz in the medium wave range and several 10 kHz in the short wave range even though a separation for the various ranges is provided, so that accurate setting of the desired station according to the provided scale is difficult.
It is known to overcome these drawbacks by providing a digital frequency indication(see for example, Funk-Technik 1971, Issue No. 5, pages 157-159 and Radio Mentor, 1973, Issue No. 9, pages 389-391).
The principle of digital frequency indication consists in that the number of oscillations of the AM or FM oscillator are counted by means of an electronic counter and are optically reproduced after subtracting the intermediate frequency. The counting process takes place periodically during a fixed period of time. For example, the oscillations are counted every 20 ms for a period of 10 ms.
The signals obtained from the FM or AM oscillator are too weak to actuate counting circuits. For this reason, in the known digital frequency indication arrangements each oscillator, i.e., the AM oscillator and the FM oscillator, has associated to it a broadband amplifier which amplifies the oscillations of the respective oscillator. The amplified oscillations are then converted to rectangular oscillations of the same fundamental frequency which are then suited to actuate the counting circuits.